It has long been known that paper, made for example on Fourdrinier wire, is significantly enhanced in its characteristics by having such paper coated with a suitable coating material. Such coating of paper, among other things, tends to make both sides of the paper of more uniform quality and enhances its use in subsequent printing processes.
The prior art has heretofore proposed various types of apparatus in an attempt to apply a uniform coating substance to the paper web in a manner whereby streaking and other undesirable characteristics are eliminated. However, none of such prior art devices have proven to be totally acceptable.
For example, U.S. of America Letters Pat. No. 3,079,889 discloses a coating apparatus having a coating applicator space which is defined, on the side where the paper web first enters it, by a slide seal formed with or against the paper web. This, of course, is a distinct disadvantage because at such point, where the slide seal exists, the paper web is still dry and therefore a relatively great friction occurs as between the slide seal and the paper web which, in turn, results in a relatively great danger of the paper web tearing.
In U.S. of America Letters Pat. No. 3,518,964 an attempt was made evidently to solve the problem of paper web tearing (as exists in the structure of said Pat. No. 3,079,899) by applying the coating substance to the paper web through an elongated chamber with a nozzle-like mouth or outlet disposed in juxtaposition to the moving paper web. In such an arrangement, the excess coating substance is scraped-off the paper web by a doctor blade. The structure of said Pat. No. 3,518,964 presents a further problem that being the entrainment of air into the coating substance. As a consequence the entrained air prevents the formation of an at least substantially smooth coating on the paper web.
U.S. of America Letters Pat. No. 4,250,211 discloses a coating apparatus wherein an applicator applies the coating substance to the paper web and a gap is provided generally adjacent the applicator on the side thereof from where the paper web enters. The applicator provides an excess of coating substance and such excess flows through such gap in a direction which is counter to the running direction of the paper web thereby forming a liquid seal which is intended to keep air out of the applicator space to prevent the entrainment of air in the substance coating the paper web. In such apparatus the excess coating substance issuing through said gap is preferably under superatmospheric pressure and of a relatively high rate of flow. However, it has been discovered that such an apparatus still fails to prevent the entrainment of air within the coating material applied to the paper web. This is believed due to the fact that the boundary layer of the air, directly at the paper web, has about the same transport velocity as the paper web itself. This causes the boundary layer of air to have a pressure in the order of about 40.0 mm. water column (400 N/m.sup.2), so that such pressure, exhibiting itself as a back-pressure against the liquid seal, enables the air to easily penetrate the liquid seal and become entrained in coating substance being applied to the paper web. It has also been discovered that another disadvantage with apparatus, in accordance with said Pat. No. 4,250,211, is the occurrence of variations in the length and/or width of the paper web actually wetted by the coating substance varies greatly. This is believed due to the use of the liquid seal or blocking stream and the action thereagainst by the said boundary layer of air.
The invention as herein disclosed and described is primarily directed to the solution of the aforestated and other related and attendant problems of the prior art.